The father of missing four-year-old Madeleine McCann said he strongly believed his daughter was alive, rejecting reports she may have been murdered after disappearing from their apartment in Portugal.

"We are not naive but on numerous occasions Portuguese police have assured us they were looking for Madeleine alive," Gerry McCann said in a televised interview with Sky News on Tuesday.

"I don't know of any information that has changed that," he said, standing next to his wife Kate.

The Diario de Noticias daily reported on Tuesday that Portuguese police had found traces of blood on the wall of the apartment where Madeleine went missing and they now fear she might have been murdered.

The newspaper quoted unnamed sources as saying: "Police have known for a month that Madeleine McCann was killed that night (May 3) at the apartment in the Praia da Luz resort."

However, it was not clear whether the blood belonged to Madeleine as tests have not yet been completed and the newspaper did not say how police had come to the conclusion that the child was murdered.

A police spokesman declined to comment on the newspaper report. But Gerry McCann said he and his wife believed Madeleine was still alive.

"Kate and I strongly believe Madeleine was alive when she was taken from the apartment," he said.

Madeleine went missing from the Praia da Luz resort in the Algarve tourist region on May 3, just yards from where her parents were dining.

Briton Robert Murat, 33, has been identified by police as the main suspect in the investigation and his property has twice been searched.

The daily also said police had identified a second suspect -- a 40-year-old white man -- and were also investigating some friends of the McCann family who were staying at the resort when the girl went missing.

Madeleine's parents have campaigned relentlessly to draw attention to her disappearance. Business tycoons and celebrities ranging from Harry Potter author J.K. Rowling to football stars have contributed to a reward for her return.

There have been a number of possible sightings of Madeleine -- from Morocco to Argentina -- since she went missing but police have come up with no concrete results.

Last week Belgian authorities said they were conducting DNA tests on a bottle and a straw after another possible sighting of the missing girl.